1. Field of the Invention
This invention relates, in general, to photocopying and, more specifically, to hardcopy output devices which utilize process control responsive to test patch measurements.
2. Description of the Prior Art
Photocopiers, electrophotographic printers, and like devices, all need the ability to maintain proper control over the image-producing apparatus to insure high quality, hardcopy outputs. One means for controlling a variety of parameters within the apparatus is by using a test area or process control patch positioned on the photosensitive or charge-retaining member of the apparatus. The test area is traditionally a small area located on the photosensitive member outside the region or frame allocated for the normal image information which will be transferred to a hardcopy output medium, such as a sheet of paper. U.S. Pat. No. 4,949,105 (Ser. No. 394,443) illustrates such a conventional patch or test area. In some cases, the test area is developed by toner and reflected light from the developed patch gives an indication of the density of the developed image and allows for changes in the operating parameters to improve the image quality. The toner concentration, the charge on the photosensitive member, the bias applied to the transfer station, and the amount of exposure from the printhead or exposing device are parameters which can be changed according to the information received from the circuitry which looks at the amount of light reflected from the test area. In other cases, the test area is sensed before development to determine what, if any, process changes need to be made.
The test patch or area is usually located outside the frame area which will be used for imaging and developing the data information for transfer to the hardcopy output sheet. However, this type of test patch placement removes the patch from the actual area where the process needs to be monitored, thus adding a possible degree of error in the monitoring and controlling process. Placing the test patch within the frame area to eliminate this possible error necessarily requires an extra frame which is not used for image information, a frame with a portion thereof dedicated to the test patch, and/or multiple-frame memory problems which can occur when the same information is imaged at the same location for several frames. The latter problem is usually caused by the memory characteristics of the photosensitive member when the same image is frequently exposed at the same location on the member.
For these reasons, a very accurate and efficient test patch has not been provided according to the prior art. Therefore, it is desirable, and an object of this invention, to provide an accurate and efficient system for maintaining the performance of hardcopy devices which can produce output sheets having several unrelated and separate images formed in a frame area and transferred to an output sheet.